1. Field of the invention
The present invention concerns controlling the torque of an asynchronous electric motor.
2. Description of the prior art
The stator of an asynchronous electric motor is usually energized by an inverter A known way to control the torque by a so-called scalar method is to operate on the stator voltage amplitude and frequency.
Controlling the amplitude of the stator voltage at constant frequency is equivalent to controlling the rotor flux, and controlling the stator frequency at constant rotor torque is equivalent to controlling the quadrature component of the stator current in a frame of reference related to the rotor flux.
It is also known to control the phase of the stator voltage relative to the rotor flux instead of controlling the stator frequency, this so-called "vector" control providing a quicker response.
A vector control system of this kind is described in French patent application N0 89 03685 corresponding to U.S. Pat. No. 5,010,288.
In this system a magnitude referred to as the set point (.alpha.cons) for the stator voltage phase relative to the rotor flux is obtained from regulated components Ud and Uq of the stator voltage respectively corresponding to the phase and quadrature components of the stator voltage in a frame of reference related to the rotor flux.
The regulated components Ud and Uq are respectively obtained from two regulators respectively receiving the rotor flux set point (.alpha.rcons) and the stator current quadrature component set point (Iqcons), the two set point values crcons and Iqcons being in turn derived from the motor torque set point.
The resulting phase set point is then compared with the current phase and if there is any difference the current stator frequency is corrected to reduce the difference.
The regulated components Ud and Uq obtained from the two regulators are also used to supply the modulus of the stator voltage to be applied to the inverter and the corrected stator frequency using a pulse width modulator to obtain the motor torque set point.
This vector control system is therefore limited to situations in which phase regulation is accompanied by modulus regulation, that is to say to situations in which the motor operates at low speeds, since it is known that modulus (i.e flux) regulation cannot be used at high speeds, when the inverter is saturated (which may be due to the minimum conduction time of the inverter power component) or when the inverter operates under so-called "full wave" conditions.